The evolution of novel herbicide tolerance in a noxious weed: the geographic mosaic of selection

Abstract

Understanding how genetic variation is organized over geography has long been of interest to evolutionary biologists given that traits can vary within and among populations, across regions, and at continental or global scales. The pattern of regional variation can have an important impact on trait evolution at the local or population level. Using a common garden, we asked whether a geographically variable mosaic of tolerance to the widely applied herbicide RoundUp® existed in two closely related co-occurring species of morning glory, Ipomoea purpurea and I. hederacea. We assayed RoundUp tolerance in over 1,700 plants representing 290 families from 29 populations in the southeastern United States. Our findings suggest that the two species of morning glory partition their respective levels of genetic variation for tolerance to glyphosate differently. Variation for tolerance in I. purpurea appears to exist among maternal lines and regions, whereas in I. hederacea, variation in tolerance existed only among populations. In addition, we find a significant hotspot of tolerance or positive spatial aggregation of this trait on a local scale in I. purpurea populations from the Coastal Plain. This suggests that either similar regimes of selection or gene flow between populations can produce a geographic mosaic of tolerance. These results highlight the fact that the genetic variation underlying an adaptive trait can exist at many different scales, whether it be within- or among-populations, among geographical ‘hotspots,’ or among distinct ecological regions. Given these results, the partitioning of genetic variation should be considered before making predictions about an adaptive trait’s evolutionary trajectory.

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Acknowledgments

The authors wish to thank V. Corby, L. Donovan, J. Estill, V. Koelling, and J. Ross-Ibarra for careful readings of the manuscript, J. Hamrick and J. Stinchcombe for helpful comments about the analysis, and J. Estill for assistance with data analysis and map making. The authors would also like to thank K. Donohue and two anonymous reviewers for thoughtful suggestions that improved the manuscript. Many thanks to the following for planting help: M. Brown, D. Gotzek, T. Haselkorn, V. Koelling, C. Richards, J. Ross-Ibarra, and J. Sterling. This project would not have been possible without the unwavering attention of M. Boyd and A. Tull in the Plant Biology Greenhouses. Funding was provided to RSB from the Mechanisms of Plant Evolution Training Grant (DBI 9602223), and an NSF DDIG (DEB-0308869).